Additionally, I'm more than adequately familiar with the gun laws of my state and know my rights, obligations, etc. Add communication skills to the mix and I'd do pretty well in front of a grand jury.

Maybe you meant jury. It would be very unlikely that you would end up in front of a grand jury. The prosecution would likely file an information and you would have a preliminary hearing. Then you would go to trial. Indictments (via grand juries) are used rarely.

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And now I will ask you why you think someone would go to jail for using a firearm LEGALLY in defense of themselves or someone else's life, but not for pounding the shit out of them with martial arts techniques, which might be construed by a county prosecutor as actively engaging and escalating a violent encounter?

The law allows people to use lethal force (roughly defined as force that has a decent probability of killing someone) in self defense or defense of others only in circumstances where that type of force is being used. Here, if the big guy punched the little guy and you pulled out your pistol at that point and shot him dead, you would almost certainly be convicted of homicide and the affirmative defense of "defense of others" would likely fail to convince anyone--since your force seemed totally disproportionate to that being used.

However, there is a hell of a lot of racism in the American criminal justice system. If you are white and you had a white Oregon jury, you may get off. In fact, the prosecutor's office may not charge you with any crime at all. If you also wealthy, you have even better chances.

If the letter of the law were followed, though, you would be convicted of homicide and possession of an illegally concealed weapon (unless you have a permit to carry it), then get extra time for using a gun in the course of a homicide. You'd be facing a lot of time.